1. Field of the Invention
This invention relates generally to the treatment of a dry eye condition, and in particular to a method and composition for this purpose which forms an artificial tear film on the surface of the eye acting to inhibit evaporation therefrom and delivering to the eye surface an efficacious medicament to treat an eye disease.
2. Status of Prior Art
The main concern of the present invention is with the treatment of a dry eye condition by a method and composition that acts to lubricate the eye and to reduce evaporation of fluid from the cornea surface. The cornea normally functions to maintain this surface in a moist and lubricated state which is impaired when the eye suffers from a dry eye condition.
Dehydration of moisture from the eye gives rise to various discomforts such an ocular dryness as well as burning and scratching sensations. But the more serious consequence of a dry eye condition is a loss of visual acuity which if it persists and is not corrected, may result in permanent damage. Dry eye disease acts to degrade the exposed ocular surface and may cause a complete breakdown of corneal tissues. In an extreme case, this may necessitate a corneal transplant.
Symptoms accompanying a dry eye condition are exacerbated when the eye is covered by a contact lens. The rate of evaporation of liquid from the eye is accelerated by the contact lens whose presence results in a meniscii formation that promotes evaporation even when the eye has an adequate natural tear film.
The usual treatment prescribed for a dry eye condition is to alleviate its symptoms by the topical application of a tear film substitute that adds a substantial volume of liquid to the anterior surface of the eye. A typical composition functioning as a tear film substitute includes soluble polymer solutions. Of prior art interest in this regard is the U.S. Pat. No. 4,421,740 to Trager which discloses an artificial tear composition formed by an aqueous hypotonic solution of lecithin, a phospholipid, and a viscosity-adjusting agent.
Of particular prior art interest are the following U.S. patents in each of which Korb is a co-inventor. Hence these patents will hereinafter be referred to as Korb patents:
I. U.S. Pat. No. 4,914,088 (1990)
II. U.S. Pat. No. 5,278,151 (1994)
III. U.S. Pat. No. 5,371,108 (1994)
IV. U.S. Pat. No. 5,294,607 (1994)
The Korb patents point out that a normal eye has an ocular surface coated with a tear film composed of:
(a) a mucous inner layer in contact with the ocular surface of the eye
(b) an aqueous middle layer which is the source of moisture, and
(c) a lipid outer layer which minimizes evaporation of the moisture from the film.
xe2x80x9cDry eyexe2x80x9d is experienced when the outer layer (c) of the tear film is defective. The dry eye treatment disclosed and claimed in Patents I to IV involves the topical application to the eye of phospholipids which form an artificial film over the eye that replicates a normal outer lipid layer and maintains the eye in moist condition.
Patent I is directed to an artificial tear film formed by:
xe2x80x9ca layer of a complex phospholipid having a net positive or negative chargexe2x80x9d.
According to this Korb patent, the significance of a net positive or negative net charge is that in either case, the charged molecules in the film coating the surface of the eye xe2x80x9crepel each otherxe2x80x9d and in doing so, maintain xe2x80x9cthe integrity of the phospholipid thereinxe2x80x9d so that it acts xe2x80x9cas a barrier reducing evaporation.xe2x80x9d Hence it is a negative or positive repelling charge that the inventor regards to be the crux of his invention.
Patent I fails to take into account that the surface of the eye being treated is anionic and therefore will interact electrostatically with a charged coating in a way that depends on the polarity of the charge. According to Patent I, the polarity of the charge doesn""t matter, for in either polarity the charged molecules in the film repel each other.
An important aspect of the present invention is not only that it has a positive net charge, but also that the strength and distribution of the charge is such as to cause the film to adhere electrostatically to the entire anionically-charged eye surface to provide an effective moisture barrier. A weak positive charge would not achieve this result. Inasmuch as in present invention, the positively-charged molecules in the film covering the eye surface electrostatically engage the negatively-charged molecules on this surface, the resultant electrostatic couple is neutral and the couples do not repel each other.
Korb patent II discloses an eye treatment composition comprising
xe2x80x9ca layer of a complex phospholipid having a net chargexe2x80x9d and
xe2x80x9ca layer of an essentially non-polar oil over said phospholipid layerxe2x80x9d, the phospholipid and oil layers being in an amount xe2x80x9cbelow that amount that would result in significant prolonged blurring of visionxe2x80x9d.
According to Patent II, the preferred phospholipids are those xe2x80x9ccarrying a net negative charge because the negatively-charged molecules would be repelled by the negatively-charged ocular surface, thereby permitting the maintenance of a relatively thick aqueous layerxe2x80x9d.
In contradistinction, the present invention which resides in a positively-charged composition, exploits the fact that the eye surface is negatively charged (anionic) so that the composition is electrostatically attracted to this surface to create a coating which prevents the escape of moisture from the eye surface for a prolonged retention period.
Korb patent III also discloses a composition in which the phospholipid has a net negative or positive charge causing molecules in the tear film coating the eye surface to repel each other to maintain the integrity of the film. In Korb patent IV, the eye treated composition is a mixture of a charged phospholipid and a non-polar oil in a meta-stable water emulsion.
Essential to the present invention is that the emulsion coating the eye surface to form a film thereon carries a net positive charge which is distributed uniformly throughout the film so that it is electrostatically attracted to the entire anionic eye surface whereby the molecules on the film surface do not repel each other but are attracted to the eye surface.
Also of particular prior art interest is PCT patent publication WO 95/31211 (Nov. 25, 1995) of Allergan, Inc. This publication discloses an emulsion for topical application to ocular tissue which includes cyclosporin admixed with castor oil. As noted in this publication, cyclosporin comprises a group of cyclic oligopeptides, the major component of which is cyclosporin A (C62H111N11O12), Clyclosporin has been found to be effective in the treatment of a dry eye condition.
In view of the foregoing, the main object of this invention is to provide an improved method and composition for treating a dry eye condition by topically applying to the eye surface an emulsion forming a tear film that adheres electrostatically to the entire surface of the eye and acts to lubricate the eye and to inhibit evaporation of moisture therefrom.
Among the significant advantages of a method and composition in accordance with the invention are the following:
A. The tear film derived from the emulsion caries a strong net positive charge that is uniformly distributed throughout the film surface whereby the film is electrostatically attracted to the entire area of the negatively-charged eye surface and there is no uncoated zone.
B. The electrostatic attraction between the artificial tear film and the eye surface maintains adhesive contact therebetween for a prolonged retention period and the tear film cannot be readily washed away.
C. The tear film coating the eye surface has no adverse effects, for the film includes no toxic or other harmful agents.
Also an object of this invention is to provide a composition of the above type which incorporates therein a therapeutic agent for treating an eye disease, such as cyclosporin A which when the composition is topically applied then delivers the agent to the eye. The release of the agent from the coating film to the surface of the eye is maintained for a prolonged period in that the film is held electrostatically in contact therewith.
Briefly stated, these objects are attained in a method and composition for treating a dry eye condition by topically applying to the eye surfaces an emulsion forming a tear film that acts to lubricate the eye and to inhibit evaporation therefrom. The emulsion is constituted by water in which is dispersed a mixture that includes a phospholipid, a non-polar oil and a polar lipid that imparts a net positive charge to the film that is distributed throughout the film, causing the film to be electrostatically attracted to the anionic surface of the eye whereby the film adheres to the eye and cannot be washed away. Includable in the mixture is a non-soluble therapeutic agent such as cyclosporin which is effective against an eye disease and is delivered to the eye by the film.